Email clients don't support every type of HTML content you see on the web. The <iframe> element is not recommended for use in emails.
An <iframe> (inline frame) embeds content from one website into another. Inline frames are often used to insert advertisements, video, audio, or forms in other websites.
iframes often contain scripts. Bad actors have abused this functionality in the past, so most email clients block them.
Instead, you can link to the content you want to display in your campaign. Ideally you would link to the content on your site to drive contacts to your website.